Plant Now, Fertilize Later

A wet early spring kept many tractors parked. Now, with corn planting dates bearing down on farmers, there may not be time to apply nitrogen fertilizer in advance.

Growers might be better off waiting to fertilize until later in the crop season, say Purdue University Cooperative Extension Service agronomists.

"Sidedressing of anhydrous ammonia this year might be the preferred route," said Tony Vyn, Extension cropping systems specialist.

Most years, about half the preplant anhydrous is applied by April 20, Vyn said. This year, excessive rain has made it difficult for farmers to do field work. Many fields have standing water or are too soft to support heavy equipment.

Unlike preplant applications, sidedressing is done after corn has emerged. The anhydrous is applied between the crop rows.

Sidedressing works with both conventional and no-till planting, and is best suited to well-drained soils. The in-season treatment offers no yield advantage but research suggests less nitrogen is lost to denitrification and leaching than with preplant applications, which are done typically weeks before seed goes into the ground.

"Sidedressing can commence as soon as farmers are able to see the corn rows," Vyn said. "It is, perhaps, most efficiently applied when corn is in about the V4 to V6 stage."

In V4, or vegetative stage four, the corn plant has four leaves with collars visible at the junction of the leaf base and stem. By V6, the plant has two more leaves with visible collars. Corn usually reaches the V4 to V6 growth stages in June.

Because the nitrogen is applied after corn has already begun to grow, sidedressing offers benefits preplant applications do not, said Vyn and Sylvie Brouder, Extension soil fertility specialist.

"The one advantage is the fact that you can apply somewhat lower overall rates of nitrogen compared to preplant applications," Vyn said. "Second, it allows for better fine-tuning of rates because it allows you the opportunity to take a pre-sidedress nitrate test and know just what your nitrogen requirements actually are."

Although nitrogen rates are often lower when sidedressing, corn makes better use of the fertilizer, Brouder said.

"If you're applying the nitrogen as a sidedress, your application will be more efficient because the nitrogen is applied closer to the time of crop need and less is lost prior to the time the crop needs the nitrogen," Brouder said. "A sidedress application will normally be equally effective at providing nitrogen to corn even when total nitrogen rates are reduced by 10 percent relative to a preplant application."

A soil nitrate test can help farmers with fields high in organic matter or a history of manure application better target how much sidedress nitrogen they'll need, if any. Farmers should collect 20-25 soil core samples at random places in a field. The samples should be air-dried and then sent to a competent soil testing lab for analysis.

"The pre-sidedress nitrate test is best done by waiting until the corn is at least 6 inches tall," Vyn said. "You should receive the results back from the lab within a matter of days and then adjust your nitrogen fertilizer rate accordingly."

While applying nitrogen fertilizer as a sidedress appears a viable option this year, potential problems can develop, Vyn said. Should the weather be uncooperative when corn reaches the ideal stage for sidedress, it may be impossible to apply the nitrogen before the crop grows too tall, he said.

Also, many farmers are squeezed for time from late May to mid-June between applying herbicides and harvesting forage, he said.

Additional information about sidedressing and nitrogen fertilizers is available in these Purdue Extension publications:

"Nitrogen Decision$ 2001: The Soil Fertility Perspective," by Brouder, Vyn and Purdue agronomists Brad Joern and Bob Nielsen. It can be downloaded online at agry.purdue.edu/ext/pubs/agry0101.pdf.